The present invention relates to an electrophotographic image forming apparatus and more particularly to a full-color digital copier, full-color printer or similar full-color image forming apparatus.
It is a common practice with a digital copier, printer or similar image forming apparatus to expose the surface of a photoconductive element in the form of dots in accordance with image data. A light source for the exposure is implemented by an LD (Laser Diode) or an LED (Light Emitting Diode) array. Usually, the apparatus forms a latent image representative of a high density image portion by selectively turning on or turning off a light beam intense enough to sufficiently attenuate the potential of the photoconductive element (saturation writing hereinafter). Also, the apparatus renders a halftone portion or a highlight portion by modulating the intensity or the duration of the light beam. Usually, to render sufficient tonality and to reproduce a highlight portion, the intensity and duration of the light beam for a single dot are made sufficiently small in order to form a latent image having a medium potential (one dot, multilevel process hereinafter). However, in electrophotography relying mainly on static electricity, the characteristic of a photoconductive element and that of a developer vary due to varying environmental conditions and aging. The problem with the one dot, multilevel process is that image quality centering around the halftone portion is susceptible to the characteristic of the photoconductive element and that of the developer.
There has been known an image forming process of the type effecting the saturation writing with all of a high density portion, a halftone portion and a highlight portion, and rendering tonality on the basis of the density of dots for a unit area (bi level process hereinafter). Basically, the bi level process does not use a medium potential portion and therefore causes a minimum of change to occur in image quality. However, dots, or unit images, output by the bilevel process are conspicuous in an image and render the image rough.
Conventional methods available for outputting high quality images, particularly halftone images, with the above image forming apparatus will be described specifically. Today, a density tonality system is extensively used which uses a recording density of 400 dpi (dots per inch) and assigns eight bits of data to each dot. The density tonality system combines PWM (Pulse Width Modulation) and PM (Power Modulation) for effecting 256-tone multi level exposure. By this exposure, a latent image having an analog medium potential is formed. The amount of toner to deposit in a unit area is varied to render tonality. This kind of scheme, also using a medium potential, is susceptible to the variation of photoconductive element and developer ascribable to environmental conditions and aging and renders a highlight portion, among others, unstable. Moreover, assigning eight bits of tonality data to each dot increases the amount of data and therefore increases the cost in relation to the increasing operation speed and recording density.
An area tonality system is another system available for rendering tonality and includes a dither method and an error scattering method by way of example. The area tonality system reduces the amount of data for a single dot, compared to the density tonality system. In addition, the area tonality system is little susceptible to the variation of the photoconductive element and developer because it does not use a medium potential. However, with the current recording density of 400 dpi, this system can implement only rough images. Stated another way, this system can stably realize high image quality if the recording density and therefore resolution is improved and if a latent image can be faithfully reproduced. This, however, cannot be done unless a latent image with a high resolution is formed by increasing the recording density and unless a developing step and an image transferring step are effected with a minimum of noise for faithful reproduction.
To achieve high image quality with an image forming apparatus of the type forming a latent image in the form of dots on a photoconductive element, Japanese Laid-Open Patent Publication No. 10-39586, for example, discusses a relation between an exposing length measured on a photoconductive element and a resolution. This document, however, relies on a highly sensitive photoconductive element which is expensive. In addition, the sensitivity of this kind of photoconductive element is apt to decrease even when the element is exposed to room light during replacement or jam processing, so that the element must be handled with the greatest possible care.
We proposed in Japanese Patent Laid-Open Publication No. 10-138566 to set up an adequate relation between the sensitivity of a photoconductive element and the duration of exposure for outputting a stable, high quality image. The adequate relation is achievable not only with a highly sensitive photoconductive element but also with a photoconductive element of ordinary sensitivity.
While the above conventional technologies are capable of improving the stability of a dot image to a noticeable degree, they give no consideration to the particle size, tinting strength and other properties of toner. Even if a latent image is formed with a high resolution, toner having a great particle size cannot faithfully reproduce the latent image and renders the resulting image rough. Further, if the tinting strength of toner is short and needs a great amount of toner to implement required image density, the toner is apt to scatter during, e.g., image transfer and degrade the resulting image. This is particularly true with a full-color image forming apparatus which sequentially transfers toner images of different colors one upon the other.
As for a high quality full-color image, Japanese Patent Laid-Open Publication No. 7-146589, for example, shows a relation between the particle size distribution of toner and the image density measured after fixation with respect to toner deposited on a recording medium in an amount of 0.5 mg/cm.sup.2. Japanese Patent Laid-Open Publication 9-54472 teaches that a full-color image with desirable color balance and color reproducibility is attained when the amounts of deposition of yellow toner, magenta toner and cyan toner each are 0.5 mg/cm.sup.2 to 0.7 mg/cm.sup.2 while the amount of deposition of black toner is 0.7 mg/cm.sup.2 to 1.0 mg/cm.sup.2.
A photoconductive element has its electrostatic characteristic deteriorated little by little due to repeated charging and exposure. It is therefore necessary to minimize the deterioration of the electrostatic characteristic by lowering a charge potential as far as possible. This is because the strength of an electric field acting on a photoconductive layer increases with an increase in charge potential, aggravating the deterioration of the photoconductive layer. However, excessively low charge potentials would make potential contrast short and would thereby lower image density and bring about background contamination. Japanese Patent Laid-Open Publication No. 10-83120 described in detail an image forming method capable of outputting desirable images even when potential contrast is low, and a developer suitable for such a method.
A digital camera, for example, is a recent achievement derived in an information networking environment and has accelerated the development of a color printer adaptive to a network. At the present stage of development, however, a full-color printer is lower in printing speed, higher in cost including a running cost and greater in size than a black-and-white printer and is not as popular as a black-and-white printer.